Well-Designed Unsymmetrical Salphen-Al Complexes: Synthesis, Characterization, and Ring-Opening Polymerization Catalysis

The unsymmetrical salphen ligand (3,5-tBu-1-OH-C6H2)­CHN-C6H4-NCH­(3-Ph-1-OH-C6H3) (tBu‑PhL­H2) was designed and synthesized to support aluminum complexes. Its symmetric analogues, (3,5-tBu-1-OH-C6H2)­CHN-C6H4-NCH­(3,5-tBu-1-OH-C6H2) (tBuLH2) and (3-Ph-1-OH-C6H3)­CHN-C6H4-NCH­(3-Ph-1-OH-C6H3) (PhLH2), were also explored and compared. The methane elimination reactions between ligands and AlMe3 resulted in formation of tBu‑PhL­Al­Me (1), tBuL­Al­Me (2), and PhL­AlMe (3) in high yields, which were characterized by elemental analysis, 1H and 13C NMR. The coordination geometries of unsymmetrical and symmetric ligands in 1 and 2 were studied by X-ray diffractions, which revealed a five-coordinated distorted square-pyramidal geometry around Al centers. The aluminum methyl compounds 1–3 reacted with BnOH at 70 °C to give tBu‑PhL­Al­OBn (4), tBuL­Al­OBn (5), and PhL­Al­OBn (6), respectively, which existed as monometallic species in solution as indicated by NMR studies. However, the aluminum isopropoxide prepared by the reaction of tBu‑PhL­H2 with 1 equiv of Al­(OiPr)3 contained three species, one monometallic tBu‑PhL­Al­OiPr (7) and two bridged dimers μ-O2-(cis-tBu‑PhL­Al­OiPr)2 (8) and μ-O2-(trans-tBu‑PhL­Al­OiPr)2 (9). The catalytic performances of unsymmetrical 4 for the ring-opening polymerization of racemic lactide (rac-LA) were preliminarily studied and compared to those of symmetric 5 and 6.